Indian Welles: World number one Rafa Nadal withstood a barrage of power serving by Croat Ivo Karlovic to reach the semi-finals of the Indian Wells ATP tournament with a thrilling 5-7, 6-1, 7-6 victory on Thursday.

The top-seeded Spaniard, champion here in 2007 and 2009, ended a tightly balanced match on the showpiece stadium court by winning the third set tiebreak 9-7 when a Karlovic forehand floated long.

Left-hander Nadal had earned his third match point of the night with a sublime inside-out forehand crosscourt winner and the crowd erupted with deafening roars when he converted it moments later to seal victory in two hours, 23 minutes.

He collapsed to the ground in delight and will next meet Argentine Juan Martin del Potro, who was gifted a place in the last four when Spaniard Tommy Robredo pulled out of their match earlier in the day with a strained left abductor muscle.

"It was an unbelievably difficult match," a beaming Nadal said courtside after beating the 6ft-10in (2.08 metre) Karlovic for a fourth time in four career meetings.

"This was the first one (of the tournament) that I felt really that even if I play well, I can lose. That`s always hard, to go on court with this feeling.”

"You lose a set against Karlovic, and you are under pressure the rest of the games of the match."

Karlovic, who became the fastest server of all time with a 251 kph (156 mph) blast during his country`s Davis Cup tie against Germany in Zagreb this month, ended the match with a total of 23 aces.

"Right now, I`m a little bit disappointed because it was close," said the 239th-ranked Croat, whose 2010 season was cut short by six months due to Achilles surgery.

"It could have also gone the other way, my way. It didn`t but, all in all, it was a good week for me, so I am happy."

The first set went with serve until Nadal was broken in the 11th game when he ran back after a protracted rally and, on the stretch, hit a forehand lob long.

Searing Groundstrokes

Karlovic held serve to clinch the set but he was broken in the second and fourth games of the second as the Spaniard began to dominate from the baseline with searing groundstrokes.

Nadal held serve to take the second set 6-1 when a Karlovic forehand service return floated long but the third went into a tiebreak with neither able to break.

The Spaniard raced into a 5-2 lead after pulling off an incredible volleyed drop shot on the stretch but the Croat, helped by a 210 kph ace, fought back to 5-5 before saving match points at 5-6 and 6-7.

"In the tiebreak I was really nervous," Nadal, the French Open, Wimbledon and U.S. Open champion, said. "I returned fantastic but I didn`t put one first serve on court."

Former US Open champion Del Potro earned his place in the last four when former world number five Robredo withdrew before their scheduled quarter-final.

The Spaniard had been troubled by his injury for a month and he aggravated it in the second set of his 6-1, 6-3 fourth-round victory over American Sam Querrey on Wednesday.

"I knew I was not going to be able to play, so that`s why I pulled out," the 25th seed said. "We did an MRI last night, and we saw the results."

Del Potro, while expressing regret for the Spaniard`s plight, looked forward to meeting Nadal in the last four.

"I am glad to be in the semi-finals," said the 22-year-old Argentine, who won his eighth ATP title at last month`s Delray Beach International Championships.

"Maybe I could play with Nadal again. We make very good matches before, so I have two days to be better, in good form, and hopefully I can play 100 percent."

In the remaining two quarter-finals on Friday, three-times champion Roger Federer will take on fellow Swiss Stanislas Wawrinka while world number three Novak Djokovic faces Frenchman Richard Gasquet.